Inflammatory bowel diseases (IBD) are associated with berrant mucosal immune responses to the enteric microflora. Innate immunity drives the active flares of disease while adaptive immune responses are thought to maintain the chronically inflamed state. While IBD has generally been associated with elevated immune responses to gut bacteria, the recent findings that IBD patients have impaired levels of innate immunity suggest that, in fact, at least some incidence of IBD may in fact result from an underlying innate immune deficiency. Thus, mechanistically dissecting how alterations in innate immunity can eventuate in chronic inflammation should help understand the pathophysiology of IBD. The bacterial protein flagellin, the monomeric subunit of flagella, is a dominant innate immune activator of intestinal epithelial cells. Thus, experimental study of the flagellin receptor, toll-like receptor 5 (TLR5), may provide mechanistic insights into how alterations in innate immunity can result in IBD. In accordance, we have recently observed that TLR5-KO mice develop spontaneous colitis. Such colitis is associated with alterations in gut microflora and appears to be dependent upon both innate and adaptive immunity. Thus, we hypothesize that TLR5 plays an essential role in managing the commensal microflora and that loss of TLR5 renders mice unable to properly manage their commensal microflora, resulting in chronic activation of other innate immune signaling pathways and development of colitogenic T-cells. Thus we propose to 1) Determine how loss of TLR5 affects immune cells and examine their role in TLR5KO colitis and 2) Define how loss of TLR5 affects the enteric microbiota and the role such changes play in driving colitis.